BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.5. Kingdom Fungi

2.5.3. Nutrition

Fungi are heterotrophs, meaning they require an organic source of carbon. In addition, they require sources of nitrogen (usually organic, such as Amino Acids), inorganic ions (such as K⁺ and Mg²⁺), Trace Elements (such as Fe, Zn, and Cu), and organic growth factors (such as Vitamins). Different fungi require a strictly specific set of nutrients, which accounts for the variety of substrates on which these fungi can be found. Fungi obtain their nutrients by absorbing them directly from the environment—unlike animals, which typically ingest food first and digest it inside their bodies, absorbing the nutrients only afterwards. When necessary, fungi are capable of extracellular Digestion. In this case, Enzymes are secreted from the fungal body onto the food source.

Based on their mode of Nutrition, fungi are classified into saprotrophs, parasites, and mutualists. In this respect, they are similar to most Bacteria.

Saprotrophs

Saprotrophs are organisms that obtain their nutrients from dead organic material. Saprotrophic fungi produce a wide range of digestive enzymes. If a saprotroph is capable of secreting digestive enzymes belonging to the three main classes—specifically, 1) carbohydrate-degrading enzymes, such as amylases (which break down starch, Glycogen, and related Polysaccharides),

2) lipases (which break down Lipids), and 3) proteinases (which break down Proteins)—then it can utilize a vast array of substrates. Penicillium species form green and blue Molds on substrates such as soil, damp leather, bread, and decaying fruit.

Hyphae of saprotrophic fungi generally exhibit positive chemotropism. In other words, they grow toward specific substrates in response to chemical substances diffusing from those substrates.

Saprotrophic fungi typically produce large quantities of lightweight, resistant spores. This enables them to easily disperse to new nutrient sources. Examples of such fungi include Mucor, Rhizopus, and Penicillium.

Saprotrophic fungi and bacteria together form the group of decomposers, which play a vital role in the global cycling of nutrients. Especially important are the few fungi that secrete cellulase and lignase, which break down Cellulose and Lignin, respectively. Since cellulose and lignin (complex compounds found primarily in wood) serve as major Structure/83.html">Structural elements of Plant Cell Walls, the decay of wood and other plant debris results partly from The activity of decomposers that secrete cellulase and lignase.

Some saprotrophic fungi are of significant economic importance. These include Saccharomyces (Yeasts), used in brewing and baking, and Penicillium (sec. 12.11.1), used in medicine.

Parasites

Parasitic fungi can be facultative or obligate (sec. 2.3.4), and they parasitize plants much more frequently than animals. Obligate parasites generally do not kill their hosts, whereas facultative parasites frequently do and subsequently live on the dead remains as saprotrophs. Obligate parasites include powdery mildews, downy mildews, rusts, and smuts. These fungi parasitize mainly cereal crops, but they also affect many other agricultural grain crops.

Once the hyphae are inside the plant, they typically begin to grow and spread between the Cells. Facultative parasites usually produce enzymes called pectinases, which digest the middle lamella between cells, causing soft tissue rot and turning it into a "mush." Then, using cellulase to digest The Cell walls, they invade the cells and kill them. The cell contents are either absorbed immediately or first digested by fungal enzymes and then absorbed. This is precisely how plants are infected by Pythium (the CAUSATIVE AGENT OF seedling damping-off) and Phytophthora (the causative agent of late blight of potato), which belong to the phylum Oomycota. According to modern views, this phylum is ancestral to true fungi and is included in the kingdom Protoctista (sec. 2.6.2).

An example of a facultative parasite that infects humans is the Yeast Candida albicans. It is a normal and generally harmless part of the human intestinal microflora, found in approximately 5% of the adult population. However, if the natural balance between the microorganisms living on or inside our bodies is disrupted—particularly through The Use of Antibiotics or prolonged steroid therapy (the side effect of which suppresses The Immune System)—the yeast growth can spiral out of control, making them pathogenic and causing a condition known as "thrush" (candidiasis). Because this fungus thrives in moist environments, it can affect the Oral Cavity (stomatitis) and the Vagina. In the latter case, vaginal discharge increases, accompanied by itching and pain during urination. This is a very common, though non-dangerous, condition that responds well to antifungal Treatment.

Mutualism (Symbiosis)

Fungi participate in The formation of two very important types of symbiotic associations: Lichens and mycorrhizae. Lichens are a symbiotic association between a fungus and an alga—either green or blue-green (cyanobacterium). Lichens typically colonize bare rocks or tree trunks; in damp forests, they may also hang from trees. It is believed that the alga supplies the fungus with organic products of Photosynthesis, while the fungus, protected from intense sunlight, is able to absorb Water and mineral salts. Furthermore, the fungus can store water, allowing lichens to grow in conditions where no other plants could survive.

A mycorrhiza is an association between a fungus and plant roots. The fungus absorbs mineral salts and water, supplying them to the tree, and in return receives organic products of photosynthesis. Mycorrhizae are discussed in greater detail in sec. 7.10.2.